The Muslim must be aware of the impure substances and should do his best to avoid them as possible as he can.

Question:

What If I found an impure substance on my clothes after offering prayer. Do I have to wash it and repeat my prayer or what?

Answer:

The Muslim must be aware of the impure  substances and should do his best to avoid them as possible as he can. The Messenger of Allah (peace be upon him) passed by two graves and said, “Both of them (the persons in these graves) are being tortured, and they are not being tortured for a cardinal sin. But indeed they are great sins. One of them used not to save himself from being soiled with his urine, and the other one used to go about with calumnies.” (Al-Bukhari and Muslim)

This hadith teaches us that the Muslim should take precautions to avoid getting any kind of impurities on himself.

Sheikh Ibn `Uthaymin  (may Allah have mercy upon him) views that with regard to a person’s clothes, the basic principle is that they are pure so long as he is not certain that some impurity has gotten onto his body or clothes. (Fatawa Ibn ‘Uthaymin, 11/107)

Responding to this question, Sheikh Ahmad Al-Hijji Al-Kurdi, an expert in the Encyclopedia of Jurisprudence and a member of the Kuwaiti Ifta department of the Ministry of Awqaf and Islamic Affairs, states:

“You have to wash your clothes, perform a new ablution, and repeat your prayer if you are certain that your clothes are affected by any kind impurities.”

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Source: Islamic-Fatwa.com